Method of induction hardening for improving fatigue strength of boundary of heated zone

ABSTRACT

Residual tensile stresses are reduced in a locally casehardened steel object by cooling the boundary zone adjacent the area to be hardened during induction heating.

United States Patent Tomita et al.

[ 51 Mar. 28, 1972 [54] METHOD OF INDUCTION HARDENING FOR IMPROVING FATIGUE STRENGTH OF BOUNDARY OF HEATED ZONE [5]] lnt.Cl. ..C2ld l/l0, C2ld 1/66 [58] Field oisearch ..l48/l45,146,148,149,150, 148/154 References Cited UNITED STATES PATENTS 2,958,524 11/1960 Delapenaetal ..148/145 2,275,402 3/1942 Crowe ..148/l49 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard AttorneyWatson, Cole, Grindle and Watson [5 7] ABSTRACT 4 Claims, 3 Drawing Figures inventors: Katsunobu Tomita; Kentaro Ishii, both of Tokyo; Yoshito Tanaka, Nara; Takao Saito, Nishinomiya, all of Japan 173] Assignees: Japam National Railways, Tokyo, Japan; Sumitomo Metal Industries, Limited, Osaka, Japan [22] Filed: Sept. 17, 1969 [21] Appl.No.: 858,812

[30] Foreign Application Priority Data Sept. 18, 1968 Japan ..43/67742 [52] U.S.Cl ..148/154 5 1 v5 2 id RES/DUAL PATENTinmzs 1972 SHEET 1 BF 2 EELS/004A STRESS QQRMMQQEQU mvmrozz U ATTORNEY PATENTEmmza 1912 3.652 346 sum 2 OF 2 ATTORNEY METHOD OF INDUCTION IIARDENING FOR IMPROVING FATIGUE STRENGTH F BOUNDARY 0F HEATED ZONE This invention relates to a method for improving the fatigue strength at the boundaries of an induction-hardened zone on a steel object, such as, for example, a straight or curved shaft for a railway car axle, an armature shaft, a mill roll or a crank shaft.

Generally, a steel object is induction-hardened to improve its resistance to fatigue and wear. However, it is known that when steel objects are locally induction-hardened, a reduction in the fatigue strength of the object will occur at the boundary areas adjacent the hardened zone and the tendency of the steel object to break at such areas will be increased.

An object of the present invention is to provide a method for preventing the reduction of the fatigue strength at such boundary areas by reducing the residual tensile stresses developed in the areas bounding a hardened zone.

A feature of the method of the present invention is the improvement of the distribution of residual stresses generated areas bounding a hardened zone by cooling such areas during induction heating so that the thermal effect may be minimized.

In the accompanying drawings:

FIG. 1 is a graph illustrating the relationship between the position of a conventional induction coil and the distribution of residual tensile stresses on the surface of a steel object which has been locally case-hardened by a conventional induction-hardening method.

FIG. 2 is an elevational, longitudinal view partly in cross section showing an embodiment of the method of the present invention.

FIG. 3 is a graph comparing the distribution of residual stresses on the surface of a shaft hardened by the conventional method and residual stresses on the surface of a shaft hardened by the method of the present invention.

Before explaining the present invention, a conventional method shall be described with reference to FIG. 1 wherein the line Y in the graph illustrates the distribution of residual stresses in a longitudinal direction along the surface of a conventionally hardened straight cylindrical steel shaft. In FIG. 1, X refers to an induction coil and Z refers to a hardened zone on the surface of the steel object (shown in dashed lines). The reference letters X, Yand Z in FIG. 1 are all shown as being disposed at corresponding axial positions. As understood from FIG. 1, at locations within the hardened zone Z, a large residual compressive stress will be developed. However, at the areas adjacent the boundaries of the hardened zone, the residual stress is reversed to develop a residual tensile stress which quickly reduces after passing a maximum. Thus, in the case of conventionally locally case-hardened steel objects, a residual compressive stress will be generated in the hardened zone itself; however, at areas adjacent the boundaries of the hardened zone, a residual tensile stress will be generated to reduce the fatigue strength of the object and increase its susceptibility to fatigue-breakage at such locations.

The method of the present invention will be explained with reference to the drawings. A shaft l, illustrated in FIG. 2, is constructedof a carbon steel (AISI 1038) containing 0.38 percent C and has a diameter of 190 millimeters. A hollow copper induction coil 2 having an inside diameter of 225 millimeters and six windings in series is disposed around the outer periphery of shaft 1. A cooling ring 4 having cooling water jetting orifices 3 is provided at each end of induction coil 2. The cooling water jetting orifices 3 are disposed on the inner periphery of cooling ring 4 so that jetted cooling water is directed onto areas b extending outwardly on shaft 1 from the boundaries of a zone a being hardened. Electric current is passed through induction coil 2 for seconds while 10 tons/hour of cooling water are jetted through jetting orifices 3. When the surface temperature of zone a has been raised to 830 C., the entire zone is quenched. The residual stresses at various positions along the surface of the hardened shaft '1 were measured with X-rays and the results are shown in FIG. 3 where the line B is a plot of residual stress against distance along the axis of the shaft. The line A in FIG. 3 is a similar plot of a case where conventional methods were used. Except for .the fact that cooling rings were not used, the conditions were accordance with the present invention have an approximately uniform value. Accordingly, the method of the present invention is very effective in improving the fatigue strength at areas adjacent the boundary of hardened zones on locally casehardened objects.

What is claimed is:

1. A method for producing a locally case-hardening steel object having improved fatigue strength characteristics comprising:

locally case-hardening a steel object by induction heating of the same at a zone thereon constituting less than the entirety of its outer surface to produce a hardened zone bounded by at least one intentionally unhardened boundary area; and

preventing the development of excessive residual tensile stresses at said boundary area by cooling the same during said case-hardening operation.

2. A method as set forth in claim 1 wherein said cooling is accomplished by directly contacting said area with a coolant fluid stream.

3. A method as set forth in claim 2 wherein said fluid is water.

4. A method as set forth in claim 2 wherein said zone and said area are generally cylindrical in shape, said coolant stream comprising a plurality of radially directed jets arranged around the object. 

2. A method as set forth in claim 1 wherein said cooling is accomplished by directly contacting said area with a coolant fluid stream.
 3. A method as set forth in claim 2 wherein said fluid is water.
 4. A method as set forth in claim 2 wherein said zone and said area are generally cylindrical in shape, said coolant stream comprising a plurality of radially directed jets arranged around the object. 